Hot-melt adhesives (hot melts) are adhesives which are based on thermoplastic polymers. These polymers are solid at room temperature but soften on heating to become viscous liquids and can therefore be applied in the form of a melt. In contrast to what are referred to as warm-melt adhesives (warm melts), which have a pastelike consistency and are applied at slightly elevated temperatures, typically in the range from 40 to 80° C., the hot-melt adhesives are applied at temperatures of 80° C. or above, typically 85° C. or above. On cooling to room temperature they solidify and at the same time develop the bond strength. Conventional hot-melt adhesives are non-reactive adhesives. On heating they soften or melt again, so making them unsuited to use at elevated temperature. Furthermore, conventional hot-melt adhesives often show a tendency, even at temperatures well below the softening point, to creep (cold flow).
These disadvantages have been largely eliminated in the case of what are called the reactive hot-melt adhesives through the introduction into the polymer structure of reactive groups which lead to crosslinking. Particularly suitable reactive hot-melt adhesives are polyurethane compositions, also referred to for short as PUR-RHM. These compositions are typically composed of polyurethane polymers which contain isocyanate groups and are obtained by reacting suitable polyols with an excess of diisocyanates. Following their application, they rapidly develop a high bond strength, by cooling, and acquire their ultimate properties, more particularly heat distortion resistance and resistance to environmental influences, through the crosslinking of the polyurethane polymer as a result of the reaction of the isocyanate groups with moisture. Owing to the carbon dioxide gas produced during the crosslinking reaction, however, there is a danger of bubbles forming within the adhesive, which may reduce the ultimate strength and the substrate adhesion and also, in the case of visible bonds, in the packaging sector for example, may adversely affect the aesthetics. Particularly prone to forming bubbles are amorphous PUR-RHM, since the skin of cured adhesive that forms from the surface is extremely impervious to carbon dioxide. At the same time the skin also allows virtually no moisture to penetrate the lower-lying, as yet uncured layers of adhesive, with the consequence that complete crosslinking takes an extraordinarily great time or does not come about at all with such adhesives.
In the field of the one-component polyurethanes which are applied at room temperature, systems which cure without bubbles are known. They typically comprise latent curing agents, more particularly aldimines. WO 2004/013200 A1 describes compositions which comprise polyaldimines, can be applied at room temperature and cure without a nuisance odour. WO 2007/036574 A1 discloses compositions which can be applied at room temperature and comprise aldimine-containing compounds which are obtainable from polyisocyanates and aldimines with groups containing active hydrogen. WO 2007/036575 A1 discloses compositions comprising polyurethane polymers that are solid at room temperature and contain aldimine groups, these compositions being suitable for use as reactive polyurethane hot-melt adhesives.
Plastics have for a long time already been bonded using hot-melt adhesives. More particularly they are adhered as a film to supports in laminations. Plastics used are often plasticizer-containing plastics, more particularly in the form of films. One plasticizer-containing plastic frequently used for technical purposes is plasticized polyvinyl chloride (pPVC). When plasticizer-containing plastics are bonded with hot-melt adhesives, however, more particularly with reactive polyurethane hot-melt adhesives, a problem very frequently evident is that the adhesion of the adhesive to the plastic is adversely affected by the plasticizer present in the plastic, and the strength of the adhesive bond begins to lessen soon after its production, and in time may lead to complete loss of adhesion. Owing to the fact that the plasticizer of the plasticizer-containing plastic in an adhesive bond with a plasticizer-free substrate may occasionally even be detected on the surface of the plasticizer-free substrate, it must be assumed that to a large extent the plasticizer migrates from the plasticizer-containing plastic into the adhesive and occasionally even completely through the adhesive. Such migration on the part of a plasticizer, however, is extremely undesirable, since on the one hand, as already mentioned, the adhesive may gradually lose its adhesion to the plastic and, on the other hand, the plasticizer content of the plastic may be greatly reduced, possibly leading to its embrittlement.